A phosphor is used in various optical devices such as display devices, illuminating devices, solar power generation devices, photonic devices, and optical amplifiers. In particular, the phosphor which is used in a wavelength conversion film or the like of a light electromotive device such as a white light emitting element (a white LED element) or a solar battery is required to emit light (fluorescence) with a high level of efficiency by electron beam excitation of comparatively low energy of near-ultraviolet rays, blue light of visible light, or the like. In addition, in the white light emitting element or the solar light electromotive device, a wavelength conversion material which is excellent in long-term stability (low degradation over a long period of time) of the phosphor, and has a small decrease in luminance efficiency under high temperatures and high humidity is required. In particular, in an optical device using a wavelength conversion material which is excited by near-ultraviolet rays or visible light and emits visible light or infrared rays, it is necessary to make high efficiency of light emission and high reliability and excellent durability of light emission compatible.
As a phosphor material which is excited by near-ultraviolet rays or blue light and emits visible light with a high level of efficiency, a YAG-based phosphor having a garnet structure such as an yttrium aluminate garnet (YAG) is generally used. In (Y, Tb, Gd)3Al5O12:Ce3+ which is an example of the YAG-based phosphor, quantum efficiency is approximately 90% at room temperature. As other phosphor materials, a phosphor powder such as silicate, borate, and phosphate has been developed and proposed, but the humidity resistance or heat resistance thereof is lower than the YAG-based phosphor. In addition, as a phosphor having the same level of luminance efficiency or durability as that of the YAG-based phosphor, a nitride or an oxynitride-based phosphor has been proposed.
These phosphors are, in general, used as a wavelength conversion member by dispersing a powdery phosphor in an organic binder such as an epoxy resin or a silicone resin, or in glass or a silica-based inorganic binder.
The refractive index of such phosphor particles having high efficiency and high reliability is approximately 1.63 to 2.0, and on the other hand, the refractive index of the organic binder or the inorganic binder is less than 1.6. Therefore, scattering occurs due to a refractive index difference between the phosphor particles and the binder. For example, in an optical element which allows excitation light to be incident from a back side of a wavelength conversion member and allows emit light to be output to a front side of the wavelength conversion member, light utilization efficiency loss due to back scattering is required to be reduced. Therefore, the particle diameter of the phosphor is approximately 10 μm.
However, in a case of the phosphor particles having a particle diameter of approximately 10 μm, absorption of the excitation light and output of the emitted light are performed only in a front layer portion of the particles, and an inner portion of the particles hardly exhibits a function as a phosphor, and thus the phosphor particles are useless as a material.
Therefore, in order to improve light utilization efficiency due to the scattering of the phosphor particles, a technology in which a front surface of the phosphor particles is covered with a porous covering layer, and thus the scattering is limited in the front surface of the phosphor particles has been proposed (PTL 1).
In addition, an approach in which the light utilization efficiency loss due to the light scattering is reduced by dispersing the phosphor particles with a particle diameter shorter than the wavelength of the excitation light or the emitted light in the binder has been also proposed. For example, a luminescence material in which rare-earth metal as a light emitting center is supported on a zeolite single crystal (PTL 2), a spherical inorganic phosphor in which oxide phosphor nano-particles are dispersed in a spherical silica matrix (PTL 3), or the like has been proposed.
Further, an approach in which light scattering properties are limited by decreasing the particle diameter of the YAG-based phosphor has been also proposed.